Sizes of Near-Earth Objects

Fragments of asteroids and comets pervade interplanetary space. Modest cosmic impacts occur all the time. On a dark, clear night one can see a flash of light (a meteor or "shooting star") every few minutes as an interplanetary grain of dust or sand strikes Earth's upper atmosphere. More rarely, larger space rocks cause brilliant "fireballs" when they crash to Earth, perhaps leaving meteorites in the ground. Every few years, Earth-orbiting surveillance satellites record multi-kiloton upper atmospheric explosions when a house-size cosmic object impacts. This happened over the Yukon Territory in January 2000, lighting up the night sky ten times more brilliantly than full daylight.

Objects 50 meters (164 feet) across strike Earth every few centuries, causing airbursts that rival the effects of large thermonuclear bombs. The last one exploded over the Tunguska region of Siberia in 1908, toppling trees over a region the size of Washington, D.C. A...